Larval cells lined with a white fungus...Asphondylia ambrosiae Gagné

Galls are whitish, hairier than the normal stem (especially in California specimens), and, because they are polythalamous, are generally larger than those of [Contarinia parthenicola].

"Forms bud-galls. Hosts: A. artemisiifolia ... [also mentions A. monophylla as a host, but Kew's POWO considers this a synonym of A. artemisiifolia.]"

"A high percentage of parasitism exists... Four adults were reared from a total of 278 galls; 6 larvae may be found in one gall. If all the larvae grew to the adult form without being parasitized, the total amount of reared adults should number 1,668 instead of 4."

A. ambrosiae forms fuzzy, globose bud galls on Ambrosia spp. Pupation takes place within the galls.